1) Field of the Invention
The invention relates to a process for the ignition of CVD plasmas in a reaction chamber for the cladding of substrates wherein a reaction gas is passed through the reaction chamber wherein the plasma, after ignition, is stimulated by means of microwave pulses and is maintained for a predetermined time span. The invention also concerns an apparatus for performing the process.
2) Background Art
For the cladding of substrates, particularly glass substrates, the latter are exposed to a plasma in a cladding chamber. Depending on the type of coating desired, appropriate reaction gases are employed which, however, differ with respect to their ignition tendency. Ignition tendency is understood to mean a low ignition voltage of the gas and/or a low extinction voltage. Microwave plasmas, in particular, exhibit the property of being difficult to ignite, especially if the plasma contains a gas acting as an electron captor. Particularly significant ignition troubles occur with gases for pulse-shaped plasmas since the plasma must be reignited after each pulse interval. Such plasmas are described, as PICVD plasmas, for example, in J. OPT. COMM. 8/1987, pages 122, et seq.
U.S. Pat. No. 4,888,088 discloses a process for the ignition of a microwave down-stream plasma wherein the reaction gases are stimulated in a chamber upstream of the reaction chamber. The ignition of this plasma takes place at the end of the reaction chamber on the gas inlet side by coupling in 1 MHz of high voltage.
The use of high frequency represents an expensive solution because the initial investment outlay for the high-frequency components generally rises with the frequency which they are intended to handle.
The placement of the ignition electrode on the gas inlet side according to U.S. Pat. No. 4,888,088 is not damaging for the application disclosed but can be disadvantageous in other microwave cladding methods, such as, for example, the PICVD method, inasmuch as the reaction gases are combined and intermixed even upstream of the substrate. Ignition of the reaction gases in the zone upstream of the reaction chamber then leads to reaction of the gases with one another; this, in turn, has the consequence that a portion of the reaction gases will be deposited already upstream of the reaction chamber. Besides, undefined reaction products, such as dust, for example, are formed, causing dullness of the layer applied to the substrate. The layers formed on the vessel walls by the partial deposition of the reaction gases on account of the ignition procedure are generally of poor adherence and peel off easily; as particles, they additionally impair the quality of the layers produced on the substrate.
A further drawback resides in that an accurate maintenance of the layer thickness is no longer possible, in spite of predetermination of the mass flow of the reaction gases, since an undetermined proportion of the reaction gases is consumed in the zone of the ignition electrode during ignition.
A process and an apparatus for the surface treatment of workpieces by corona discharge has been known from DOS 3,322,341. In order to avoid ground discharges and thus damage to the workpiece, on the one hand, and to prevent reoccurrence of ignition problems, on the other hand, the voltage pulses exhibit in each case in the initial zone a pulse peak for igniting the corona discharge and subsequently pass over into a region with an amplitude suitable for maintaining the corona discharge. This method, making it possible to carry out cladding, hardening, annealing, and the like, differs from processes working with microwave plasmas in that the workpiece is connected as the cathode and the wall of a vacuum vessel surrounding the workpiece is connected as the anode to a voltage source of several 100 to 1,000 volt. However, this conventional process cannot be utilized for the cladding of glass substrates, for example.